1. Field of the Invention
This invention is directed to a method and apparatus for improving the performance of disk drive recording systems. In particular this invention is directed to reducing the delay between successive read/write operations caused by the necessity of moving the recording head from a current track to the track for the next read/write operation. This delay is referred to as track switching latency.
2. Description of Prior Art
In the past track switching latency has been dealt with in a limited way by making use of two or more recording head actuators distributed about the recording disk circumference. So long as the next read/write operation used a recording head on an actuator other than the currently active actuator, the next recording actuator could be moving the head to the correct track position while the first head finishes the current read/write operation.
One example of the above multiple actuator technique is taught in U.S. Pat. No. 4,270,154 issued to John E. Crawford. The Crawford patent is directed to overlapping the operation of one read/write head actuator with a second read/write head actuator so as to reduce system latency between read/write operations. In particular Crawford teaches algorithms for selecting the recording head and actuator for the next read/write operation so as to reduce latency. One of Crawford's algorithms evaluates the possibility of selecting a head on the inactive actuator so that the actuator may be moved to the next desired track or cylinder while the current read/write operation by the active actuator is being completed. Crawford recognizes the limitation of his design in that if the best head for the next recording operation is on the active actuator then it is not possible with Crawford's technique to reduce track switching latency.
In other words, Crawford has no solution for the problem of reducing track switching latency when only one read/write head actuator is used or when in a multiple actuator disk drive, the track switching is between heads on the same actuator. Unfortunately, most track switching in an optimized storage routine is track switching between tracks on the same cylinder; i.e., between heads on the same actuator.
Track switching between tracks on the same cylinder is no longer simply a matter of selecting another head in a single stack of heads on the same actuator. As track density has increased, thermal expansion and other mechanical characteristics of the disk drive result in different alignment positions for each head in a stack with its corresponding track. Therefore, the head actuator carrying the stack of heads must be moved when track switching occurs even between tracks in the same cylinder. In other words, what in the past might have been simply a head selection operation to switch between tracks on the same cylinder now requires actuation of the head actuator to move the new head into proper alignment with the new track in the same cylinder.